1. Field of the Invention
This invention relates to a method for producing a catalyst for the hydration of olefins from clay minerals.
2. Description of the Prior Art
It is known that olefins in the gaseous phase can be converted under high pressures with steam into alcohols. Such methods have special technical significance in the production of ethyl alcohol from ethylene and isopropyl alcohol obtained from propylene. These alcohols are synthesized in the presence of catalysts, usually phosphoric acid provided on carriers.
Suitable carrier materials are based on pure silicic acid (e.g. diatomaceous earth or silica gel) or on silicic acid with greater or lesser clay content, such as calcined diatomaceous earth, whose structure is held together by clay or clay-like materials.
Long-term solidity is a problem with carrier materials based on pure silicic acid. The clay-containing materials have better mechanical solidity, however, they have the disadvantage that when their clay-content is too high aluminum oxide is dissolved away during the reaction due to the action of the phosphoric acid.
German Pat. No. 1 156 772 describes a method for producing a clay-containing carrier for phoshoric acid used as a catalyst in olefin hydration, in which molded contact bodies made of mineral clay silicates are treated with mineral acid so that the aluminum oxide content falls preferrably to between 1 and 5% by weight (b.w.). This material generally has the necessary mechanical solidity and a sufficiently low residual aluminum oxide content to avoid dissolving away. In contrast it has been observed with the use of customary contact bodies in the production of catalyst carriers for olefin hydration, that without preselecting the raw material, strongly differing catalyst activities are produced.
Success was finally found in developing carriers for phosphoric acid on the basis of large pore silica gels with high hydration activity and adequate mechanical solidity, e.g. DE-OS No. 26 25 705 and DE-OS No. 27 19 055. Nonetheless, one disadvantage remained with these carriers based on amorphous silicic acid: upon extended exposure to the rigors of the hydration reaction, the amorphous silicic acid crystallised into cristobalite and quartz. This brought with it an irreversible, stark reduction of specific surface area and thus, catalytic activity, along with a decrease in mechanical solidity. Another disadvantage of all previously used hydration catalysts based on phosphoric acid on silicate carriers is the slow reduction of activity owing to dissipated phosphoric acid. Phosphoric acid must be continuously neutralized by alkali treatment in continuous operation in order to avoid the corrosive actions of acid and raw alcohol on the down stream distillation apparatus.
The more recent development of continuous spraying of washed-away phosphoric acid according to DE-OS No. 26 58 946 made it possible to avoid, to a large extent, the continuous loss of activity and, thus, to considerably extend the life span of the catalyst. This, however, places corresponding demands on the life span of the carrier, thus eliminating the use of such carriers as those with which crystallization occurs under reaction conditions, thereby irreversibly lessening the catalytic activity and the mechanical solidity in the course of time.
German patent application No. P 29 08 491.1 shows that a carrier for a hydration catalyst with continuing high catalytic activity can be produced from clay material when care is taken in the selection of the raw material to assure that the material consists largely of montmorillonite, which means that after being formed, macerated (leached) and impregnated, the active surface area on which the hydration of the olefins can take place is large.
German patent application No. P 29 08 491.1 relates to a method for producing a catalyst from clay minerals for the hydration of olefins with 2-3 C-atoms to the corresponding alcohols of phosphoric acid and carrier material--including the correspondingly produced catalyst--in which a clay containing essentially montmorillonite, contaminated by no more than 3% accompanying minerals such as quartz, feldspar and mica, and containing up to 0.5% of K.sub.2 O, is processed in a first step with acid until it has an Al.sub.2 O.sub.3 -content of 13-18% b.w. and, if necessary, the Al.sub.2 O.sub.3 --content is adjusted to 16-18% b.w. through the addition of precipitated clay. The result is a surface area of 200-400 m.sup.2 /g, preferrably 240-300 m.sup.2 /g. When the total water content is 20-35% it is pressed into a form, calcined at 500.degree.-800.degree. C., and the formed carrier material is then treated with acid in a second step until the Al.sub.2 O.sub.3 -content reaches 1-5% b.w., preferrably 1-3% b.w. The result is a surface area of 150-250 m.sup.2 /g, preferrably 180- 220 m.sup.2 /g. Finally, the resulting carrier is impregnated in a known manner with phosphoric acid.
A different mineral in the montmorillonite group, which nonetheless, contains the montmorillonite crystalline lattice, can be used instead of montmorillonite.
It is also possible to use a fuller's earth or clay that has already been treated once with acid in place of a clay containing montmorillonite that has not yet been treated with acid. This makes the first acid treatment superfluous. This fuller's earth or clay should contain less than 0.1% K.sub.2 O; the weight ratio should be (Al.sub.2 O.sub.3 +Fe.sub.2 O.sub.3): SiO.sub.2 =1:3.5-1:4.5. If needed, the Al.sub.2 O.sub.3 --content in the fuller's earth or clay can be brought to the necessary 16-18% b.w. by adding precipitated clay.
Catalyst or catalyst carriers produced in this way from clay containing montmorillonite have a different origin than those formed contact bodies based on mineral clay silicates, increased activity, i.e. per hour and 1 catalyst charging approximately 105-110 g of ethanol or ca. 300 g of isopropyl alcohol were produced. This increased activity, however, can only be sustained over an extended period if the phosphoric acid that is carried away--amounting to ca. 0.07 g per hr and 1 catalyst charging with ethanol--is constantly balanced by continuous replacement of an equal amount of acid. In addition, this dissipated acid must still be neutralized with an alkali treatment.
The mechanical solidity of the catalysts lies in the range of 7-9 kg/ball, which is sufficient to charge a customary reactor.